"This should make us able to perform “proton astronomy” if the reconstruction errors on the direction of the incident particles are less than this deviation 1."[6]

"The prospects for future cosmic-ray investigations at superhigh energies are outlined and the great importance of development in the neutron and proton astronomy for energies above 1018-1019 eV is pointed out. [...] These factors adversely affect the investigation of [proton cosmic rays] PCR at ultrahigh energies. At the same time the situation is not utterly hopeless, as new interesting possibilities (such as neutron and proton astronomy) are opened up in this energy region."[7]

"Space radiation may be classified according to origin as: (i) galactic cosmic radiation (87% protons, 12% alfa, 1% HZE), with energies between 1 and 103 GeV; (ii) solar particle radiation, consisting of charged particles in large clouds, mainly protons with an energy of about 1 GeV; (iii) geomagnetically trapped particle radiation, generated from the interaction of the radiation with the geomagnetic field comprising electrons with energies up to 7 MeV, protons with energies up to 600 MeV, and low energy heavy ions."[8]

Nucleon spin structure describes the partonic structure of proton intrinsic angular momentum (spin). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its [suggested] constituent partons (quarks and gluons). In the late 1980s, the European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks is not sufficient to account for the total spin of [protons]. This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred to as the "proton spin crisis".

Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the COMPASS experiment at CERN, experiments E154 and E155 at [SLAC National Accelerator Laboratory] SLAC, HERMES at DESY, experiments at [Thomas Jefferson National Accelerator Facility] JLab and RHIC, and others. Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin [may] contribute about 30% to the total spin of the nucleon.

New measurements performed by European scientists reveal that the radius of the proton is 4 percent smaller than previously estimated.[9]

The free proton is stable and is found naturally in a number of situations. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay, and also result from the decay of free neutrons, which are unstable. In all such cases, protons must lose sufficient velocity and (kinetic energy) to allow them to become associated with electrons, since this is a relatively low-energy interaction. However, in such an association, the character of the bound proton is not changed, and it remains a proton.

At right is a graph or block diagram that shows the boundaries for nuclear particle stability. The boundaries are conceptualized as drip lines. The nuclear landscape is understood by plotting boxes, each of which represents a unique nuclear species, on a graph with the number of neutrons increasing on the abscissa and number of protons increasing along the ordinate, which is commonly referred to as the table of nuclides, being to nuclear physics what the more commonly known periodic table of the elements is to chemistry. However, an arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus, and ultimately when continuing to add more of the same type of nucleons to a given nucleus, the newly formed nucleus will essentially undergo immediate decay where a nucleon of the same isospin quantum number (proton or neutron) is emitted; colloquially the nucleon has 'leaked' or 'dripped' out of the target nucleus, hence giving rise to the term "drip line". The nucleons drip out of such unstable nuclei for the same reason that water drips from a leaking faucet: the droplet, or nucleon in this case, sees a lower potential which is great enough to overcome surface tension in the case of water droplets, and the strong nuclear force in the case of proton emission or alpha decay. As nucleons are quantized, then only integer values are plotted on the table of isotopes, indicating that the drip line is not linear but instead looks like a step function up close.

The general location of the proton drip line is well established. For all elements occurring naturally on earth and having an odd number of protons, at least one species with a proton separation energy less than zero has been experimentally observed. Up to germanium the location of the drip line for many elements with an even number of protons is known, but none past that point are listed in the evaluated nuclear data. There are a few exceptional cases where, due to nuclear pairing, there are some particle-bound species outside the drip line, such as 8B and 178Au. One may also note that nearing the magic numbers, the drip line is less understood. A compilation of the known first unbound nuclei beyond the proton drip line is given below, with the number of protons, Z and the corresponding isotopes, taken from the National Nuclear Data Center.[10]

The antiproton (p, pronounced p-baer) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy.

Antiprotons have been detected in cosmic rays for over 25 years, first by balloon-borne experiments and more recently by satellite-based detectors. The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction, where A represents a nucleus:

p + A → p + p + p + A

The secondary antiprotons (p) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium. The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[12]

In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of charge-to-mass ratio (e/m), they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson.

In the [Large Hadron Collider] LHC proton bunches also produce [synchrotron] radiation at increasing amplitude and frequency as they accelerate with respect to the vacuum field, propagating photoelectrons, which in turn propagate secondary electrons from the pipe walls with increasing frequency and density up to 7x1010. Each proton may lose 6.7keV per turn due to this phenomenon.[13]

Ice cores contain thin nitrate-rich layers that can be analyzed to reconstruct a history of past events before reliable observations; [this includes] data from Greenland ice cores[15] and others. These show evidence that events of [the magnitude of the solar storm of 1859—as measured by high-energy proton radiation, not geomagnetic effect—occur approximately once per 500 years, with events at least one-fifth as large occurring several times per century.[16] Less severe storms have occurred in 1921 and 1960, when widespread radio disruption was reported.

"These authors proposed that the whole-disk surface colors of KBOs could be the result of the competition between the effects of irradiation of surface organics by cosmic-rays and the global resurfacing due to impacts. [...] When these high-energy protons collide with an icy target, they penetrate very [deep] under the surface."[17]

"How is it possible that the infrared spectra of these minerals [olivine (Mg2SiO4 and enstatite (MgSiO3)] are essentially unchanged after proton bombardment? ... The lack of change following irradiation is easily understood for olivine; this orthosiicate mineral possesses an 'island' structure of isolated tetrahedra."[18]

Def. a collimated stream, spurt or flow of liquid or gas or plasma in a narrow cone of particles is called a jet.

Def. at, or near the speed of light is called relativistic.

"The structure of relativistic jets in [active galactic nuclei] AGN on scales of light days reveals how energy propagates through jets, a process that is fundamental to galaxy evolution."[19]

Their lengths can reach several thousand[20] or even hundreds of thousands of light years.[21] The hypothesis is that the twisting of magnetic fields in the accretion diskcollimates the outflow along the rotation axis of the central object, so that when conditions are suitable, a jet will emerge from each face of the accretion disk. If the jet is oriented along the line of sight to Earth, relativistic beaming will change its apparent brightness. The mechanics behind both the creation of the jets[22][23] and the composition of the jets[24] are still a matter of much debate; it is hypothesized that the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion.

"Deuterated isotopomers of methanol have been detected both in hot cores and in the protostellar source IRAS 16293-2422. [...] In studying the post-evaporative gas-phase chemistry of these isotopomers, it is important to know if pairs of isotopomers with D atoms in different places (eg CH3OD and CH2DOH) can be interconverted or whether they can be viewed as separate entities with depletion mechanisms that are independent of each other. Here we show that it is difficult to exchange protons and deuterons on the two different parts of the methanol backbone."[25]

Mesons are hadronic subatomic particles, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometre, which is about 2⁄3 the size of a proton or neutron.

In cosmic ray interactions, for example, such particles are ordinary protons and neutrons. Mesons are also frequently produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles.

If there was no nuclear force, all nuclei with two or more protons would fly apart because of the electromagnetic repulsion.

All isotopes that contain an odd number of protons and/or of neutrons have an intrinsic magnetic moment and angular momentum, in other words a nonzero spin just as electrons pair up in atomic orbitals, so do even numbers of protons or even numbers of neutrons (which are also spin-1⁄2 particles and hence fermions) pair up giving zero overall spin. A proton and neutron will have lower energy when their spins are parallel, not anti-parallel, since this parallel spin alignment does not infringe upon the Pauli Exclusion Principle, a proton is of spin 1/2. The NMR absorption (radio) frequency for tritium is however slightly higher than that of 1H because the tritium nucleus has a slightly higher gyromagnetic ratio than 1H.

About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. Solitary electrons constitute much of the remaining 1%.

Aluminium-26, 26Al, is a radioactive isotope of the chemical element aluminium, decaying by either of the modesbeta-plus or electron capture, both resulting in the stable nuclide magnesium-26. The half-life of 26Al is 7.17×105 years. This is far too short for the isotope to survive to the present, but a small amount of the nuclide is produced by collisions of argon atoms with cosmic ray protons.

Protons are known to transform into neutrons through the process of electron capture (also called inverse beta decay). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is:

The diagram contains the reactions in the proton-proton chain including neutrino production. Credit: Dorottya Szam.

The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV. [27]

If irradiated with select projectile nuclei [or protons] at kinetic energies Ekin these target elements can undergo a nuclear reaction under resonance conditions for a sharply defined resonance energy. The reaction product is usually a nucleus in an excited state which immediately decays, emitting ionizing radiation such as protons or gamma rays.

To obtain depth information the initial kinetic energy of the projectile nucleus (which has to exceed the resonance energy) and its stopping power (energy loss per distance traveled) in the sample has to be known. To contribute to the nuclear reaction the projectile nuclei have to slow down in the sample to reach the resonance energy. Thus each initial kinetic energy corresponds to a depth in the sample where the reaction occurs (the higher the energy, the deeper the reaction).

The energetic emitted γ ray is characteristic of the reaction and the number that are detected at any incident energy is proportional to the concentration at the respective depth of [nitrogen] in the sample. The N concentration profile is then obtained by scanning the proton incident or transmitted beam energy.

NRA can also be used non-resonantly. For example, deuterium can easily be profiled with a 3He beam [or 3He with a deuterium beam] without changing the incident energy by using the

3He + D = α + p+ + 18.353 MeV

reaction. The energy of the fast proton detected depends on the depth of the deuterium [or 3He] atom in the sample.

Proton-induced X-ray emission (PIXE) is a technique used in the determining of the elemental make-up of a material or sample. When a material is exposed to [a proton] beam, atomic interactions occur that give off [X-rays] specific to an element.

In addition to the X-ray spectrum, a Rutherford backscattering spectrum, and a proton transmission spectrum may be collected.

Bombardment with usually MeV protons produced by an accelerator, cause inner shell ionization of atoms. Outer shell electrons drop down to replace inner shell vacancies. Characteristic X-rays for each element are emitted. Either an energy dispersive or a wavelength dispersive detector is used to record and measure these X-rays.

Only elements heavier than fluorine can be detected. The lower detection limit for a PIXE beam is given by the ability of the X-rays to pass to the X-ray detector. The upper limit is given by the ionisation cross section and the probability of the K electron shell ionisation. This is maximal when the velocity of the proton matches the velocity of the electron where 3 MeV proton beams are optimal.

Protons can also interact with the nucleus of an atom through elastic collisions, Rutherford backscattering, often repelling the proton at angles close to 180 degrees. The backscattered protons give information on the sample thickness and composition. The bulk properties allow for the correction of X-ray photon loss within.

Proton transmission, absorption, reflection, diffraction, and emission may give information about the material.

"As a result of ion irradiation a modification (evolution) of the original target can be generated and new materials can be produced. Bonds in the target material may in fact be broken by energy deposition around the "hot" track of incoming ions. The recombination of fragments produces new and also complex molecules. If carbon is present in the target, even long chain polymer-like substances that are stable above room temperature can be produced ... The presence of organic material in the [solar system], possibly produced by bombarding ions ... seems now well supported by recent findings from space missions as from Voyager at the Uranian system".[29]

The "colour of the organic layers [synthesized organic samples of frozen CH4 (T ~ 10 K) ice] depends on the amount of energy deposited by the bombarding beam [~ 1016 protons (1.5 MeV) cm-2 and ~ 1017 protons cm-2]. When first extracted after a (relatively) low ion fluence the materials appear yellow, becoming darker and darker if again bombarded at higher doses."[29]

"The evolution of organics to carbonaceous material induced by ion irradiation is ... a well established phenomenon independent of the type of original carbon containing material."[29]

This image is a near-infrared, colour-coded composite image of a sky field in the south-western part of the galactic star-forming region Messier 17. Credit: European Southern Observatory.

The cosmic infrared background (CIB) causes a significant attenuation for very high energy protons through inverse Compton scattering, photopion and electron-positron pair production.

At right "is a near-infrared, colour-coded composite image of a sky field in the south-western part of the galactic star-forming region Messier 17. In this image, young and heavily obscured stars are recognized by their red colour. Bluer objects are either foreground stars or well-developed massive stars whose intense light ionizes the hydrogen in this region. The diffuse light that is visible nearly everywhere in the photo is due to emission from hydrogen atoms that have (re-)combined from protons and electrons. The dark areas are due to obscuration of the light from background objects by large amounts of dust — this effect also causes many of those stars to appear quite red. A cluster of young stars in the upper-left part of the photo, so deeply embedded in the nebula that it is invisible in optical light, is well visible in this infrared image. Technical information : The exposures were made through three filtres, J (at wavelength 1.25 µm; exposure time 5 min; here rendered as blue), H (1.65 µm; 5 min; green) and Ks (2.2 µm; 5 min; red); an additional 15 min was spent on separate sky frames. The seeing was 0.5 - 0.6 arcsec. The objects in the uppermost left corner area appear somewhat elongated because of a colour-dependent aberration introduced at the edge by the large-field optics. The sky field shown measures approx. 5 x 5 arcmin 2 (corresponding to about 3% of the full moon). North is up and East is left."[32]

"Radio observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA (BEMRAK) [...] at submillimeter wavelengths [show an impulsive component that] starts simultaneously with high-energy (>200 MeV nucleon−1) proton acceleration and the production of pions. The derived radio source size is compact (≤10"), and the emission is cospatial with the location of precipitating flare-accelerated >30 MeV protons as seen in γ-ray imaging."[33]

"In astronomy, interplanetary scintillation refers to random fluctuations in the intensity of radio waves of celestial origin, on the timescale of a few seconds. It is analogous to the twinkling one sees looking at stars in the sky at night, but in the radio part of the electromagnetic spectrum rather than the visible one. Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the electron and protons that make up the solar wind."[34]

"The solar wind is a plasma, composed primarily of electrons and lone protons, and the variations in the index of refraction are caused by variations in the density of the plasma.[35] Different indices of refraction result in phase changes between waves traveling through different locations, which results in interference. As the waves interfere, both the frequency of the wave and its angular size are broadened, and the intensity varies.[36]"[34]

"The tachyonic spectral densities generated by ultra-relativistic electrons in uniform motion are fitted to the high-energy spectra of Galactic supernova remnants, such as RX J0852.0−4622 and the pulsar wind nebulae in G0.9+0.1 and MSH 15-52. ... Tachyonic cascade spectra are quite capable of generating the spectral curvature seen ... Estimates on the electron/proton populations generating the tachyon flux are obtained from the spectral fits"[37]

The top image is the plum pudding model of atoms. In the bottom image are deflections. Credit: Fastfission.

For the determination of the elemental composition of liquid proteins microPIXE can quantify the metal content of protein molecules with a relative accuracy of between 10% and 20%.[38] In part by the X-ray emission from sulfur and the phosphate groups but excessive amounts of chlorine overlap with the sulfur peak; whereas KBr and NaBr do not.

In the image at right, the top image is the plum pudding model of atoms undisturbed by penetrating protons. In the bottom image, some of the protons are deflected.

There are many advantages to using a proton beam over an electron beam:

There is less crystal charging from Bremsstrahlung radiation, although there is some from the emission of Auger electrons,

there is significantly less than if the primary beam was itself an electron beam, and

because of the higher mass of protons relative to electrons, there is less lateral deflection of the beam.

Rutherford backscattering spectrometry (RBS) is an analytical technique sometimes referred to as high-energy ion scattering (HEIS) spectrometry. RBS is used to determine the structure and composition of materials by measuring the backscattering of a beam of high energy protons or ions impinging on a piece of material such as a dust grain.

If the energy of the incident proton is increased sufficiently, the Coulomb barrier is exceeded and the wavefunctions of the incident and struck particles overlap. This may result in nuclear reactions in certain cases, but frequently the interaction remains elastic, although the scattering cross-sections may fluctuate wildly as a function of energy. This case is known as "Elastic (non-Rutherford) Backscattering Spectrometry" (EBS).

We can describe Rutherford backscattering as an elastic (hard-sphere) collision between a high kinetic energy proton from the incident beam (the projectile) and a stationary particle located in the dust grain (the target). Elastic in this context means that no energy is either lost or gained during the collision.

In some circumstances a collision may result in a nuclear reaction, with the release of considerable energy. Nuclear reaction analysis (NRA) is very useful for detecting light elements.

The energy E1 of the scattered projectile is reduced from the initial energy E0:

where particle 1 is the projectile, particle 2 is the target nucleus, and θ1{\displaystyle \theta _{1}} is the scattering angle of the projectile in the laboratory frame of reference (that is, relative to the observer). The plus sign is taken when the mass of the projectile is less than that of the target, otherwise the minus sign is taken.

To describe the probability of observing such an event. For that we need the differential cross-section of the backscattering event:

where Z1{\displaystyle Z_{1}} and Z2{\displaystyle Z_{2}} are the atomic numbers of the incident [proton] and target [nucleus]. [From] the centre of mass frame of reference and is therefore not a function of the mass of either the projectile or the target nucleus.

The "scattering angle θ1{\displaystyle \theta _{1}} is not the same as the scattering angle θ{\displaystyle \theta } (although for RBS experiments they are usually very similar).

A scattering cross-section is zero implies that the projectile never comes close to the target, nor penetrates the electron cloud surrounding the nucleus. The pure Coulomb formula for the scattering cross-section shown above must be corrected for this screening effect, which becomes more important as the energy of the projectile decreases.

While large-angle scattering only occurs for protons which scatter off target nuclei, inelastic small-angle scattering can also occur off the sample electrons. This results in a gradual decrease in protons which penetrate more deeply into the sample, so that backscattering off interior nuclei occurs with a lower "effective" incident energy. The amount by which the ion energy is lowered after passing through a given distance is referred to as the stopping power of the material and is dependent on the electron distribution. This energy loss varies continuously with respect to distance traversed, so that stopping power is expressed as

For high energy stopping power is usually proportional to Z2E{\displaystyle {\frac {Z_{2}}{E}}}.

Stopping power or, stopping force has units of energy per unit length. It is generally given in thin film units, that is eV /(atom/cm2) since it is measured experimentally on thin films whose thickness is always measured absolutely as mass per unit area, avoiding the problem of determining the density of the material which may vary as a function of thickness. Stopping power is now known for all materials at around 2%, see http://www.srim.org.

When a beam of protons with parallel trajectories is incident on a target atom, scattering off that atom prevents or blocks collisions in a cone-shaped region "behind" the target relative to the beam. This occurs because the repulsive potential of the target atom bends close ion trajectories away from their original path. The radius of this blocked region, at a distance L from the original atom, is given by

When a proton is scattered from deep inside a sample, it can then re-scatter off a second atom, creating a second blocked cone in the direction of the scattered trajectory. This can be detected by carefully varying the detection angle relative to the incident angle.

Channeling is observed when the incident beam is aligned with a major symmetry axis of the crystal. Incident protons which avoid collisions with surface atoms are excluded from collisions with all atoms deeper in the sample, due to blocking by the first layer of atoms. When the interatomic distance is large compared to the radius of the blocked cone, the incident protons can penetrate many times the interatomic distance without being backscattered. This can result in a drastic reduction of the observed backscattered signal when the incident beam is oriented along one of the symmetry directions, allowing determination of a sample's regular crystal structure. Channeling works best for very small blocking radii, i.e. for protons.

The tolerance for the deviation of the [proton] beam angle of incidence relative to the symmetry direction depends on the blocking radius, making the allowable deviation angle proportional to

While the intensity of an RBS peak is observed to decrease across most of its width when the beam is channeled, a narrow peak at the high-energy end of a larger peak will often be observed, representing surface scattering from the first layer of atoms. The presence of this peak opens the possibility of surface sensitivity for RBS measurements.

1H, the most commonly used spin ½ nucleus in NMR investigation, has been studied using many forms of NMR. Hydrogen is highly abundant, especially in biological systems. It is the nucleus most sensitive to NMR signal (apart from 3H which is not commonly used due to its instability and radioactivity). Proton NMR produces narrow chemical shift with sharp signals. Fast acquisition of quantitative results (peak integrals in stoichiometric ratio) is possible due to short relaxation time.

"Proton NMR ( Hydrogen-1 NMR, or 1H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules.[43] In samples where natural hydrogen (H) is used, practically all of the hydrogen consists of the isotope 1H (hydrogen-1; i.e. having a proton for a nucleus). A full 1H atom is called protium.

This graph displays the flux of high energy protons measured by GOES 11 over four days from November 2, 2003, to November 5, 2003. Credit: NOAA.

The Sun, the coronal cloud around it, and the solar wind, which originates through the polar coronal holes apparently from the photosphere, are major sources of protons within the solar system.

"The third largest solar proton event in the past thirty years took place during July 14-16, 2000, and had a significant impact on the earth's atmosphere."[44]

At right is a temporal distribution of solar proton flux in units of particles cm-2 s-1 sr-1 as measured by GOES 11 over the four days from November 2, 2003, to November 4, 2003, in three windows of energy: ≥ 100 MeV (green), ≥ 50 MeV (blue), and ≥ 10 MeV (red).

The first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. This line results from the formation of deuterium via the union of a neutron and proton; in a solar flare the neutrons appear as secondaries from interactions of high-energy ions accelerated in the flare process. These first gamma-ray line observations were from OSO-3 [and] OSO-7.

The Bastille Day Flare or Bastille Day Event was a powerful solar flare on July 14, 2000, occurring near the peak of the solar maximum in solar cycle 23.[46][47] [NOAA] Active region 9077 produced an X5.7-class flare, which caused an S3 radiation storm on Earth fifteen minutes later as energetic protons bombarded the ionosphere.[46][48] It was the biggest solar radiation event since 1989.[48] The proton event was four times more intense than any previously recorded since the launches of SOHO in 1995 and ACE in 1997.[46] The flare was followed by a full-halo coronal mass ejection[46] and a geomagnetic super storm on July 15-16. The extreme level, G5, was peaked in late hours of July 15.

The McMath region Number 8461 passed over the solar disk during the 1966 Proton Flare Project period, from August 21 to September 4, and produced two important solar particle events on August 28 and September 2.[49]

"During the Mercury flyby of Mariner 10, observations of large fluxes of energetic ... protons (0.53 < E < 1.9 MeV) have been reported"[50] but these may be due to "the pileup of low-energy electrons rather than the presence of protons in the vicinity of Mercury."[50]

The Energetic Particles Experiment aboard Mariner 10 "was designed to measure energetic ... protons ... in the interplanetary medium and in the vicinities of Venus and Mercury. The instrumentation consisted of a main telescope and a low-energy telescope. The main telescope consisted of six co-linear sensors (five silicon detectors and one CsI scintillator) surrounded by a plastic scintillator anti-coincidence cup. One pulse height analysis was performed every 0.33 s, and counts accumulated in each coincidence/anti-coincidence mode were measured every 0.6 s. Particles stopping in the first sensor were protons ... in the range 0.62--10.3 MeV/nucleon ... . The aperture half angle for this mode was 47 degrees, and the geometric [factor was] 7.4 sq cm-sr for protons ... . The telescope aperture half-angle decreased to 32 degrees for coincident counts in the first and third sensors. The low-energy telescope, a two-element (plus anti-coincidence) detector with a 38 degree half angle aperture and a 0.49 sq cm sr geometrical factor, was designed to measure 0.53--1.9 MeV and 1.9--8.9 MeV protons without responding to electrons over a wide range of electron energies and intensities."[51]

"The ... solar proton flare on 20 April 1998 at W 90° and S 43° (9:38 UT) was measured by the GOES-9-satellite (Solar Geophysical Data 1998), as well as by other experiments on WIND ... and GEOTAIL. Protons were accelerated up to energies > 110 MeV and are therefore able to hit the surface of Mercury."[52]

Proton influx has effects on the Earth long before protons impinge on the planet's solid or liquid surface.

The Van Allen radiation belt is split into two distinct belts, with energetic electrons forming the outer belt and a combination of protons and electrons forming the inner belts. In addition, the radiation belts contain lesser amounts of other nuclei, such as alpha particles.

The trapped particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic.

While protons form one radiation belt, trapped electrons present two distinct structures, the inner and outer belt. The inner electron Van Allen Belt extends typically from an altitude of 1.2 to 3 Earth radii (L values of 1 to 3).[53] In certain cases when solar activity is stronger or in geographical areas such as the South Atlantic Anomaly (SAA), the inner boundary may go down to roughly 200 kilometers[54] above the Earth's surface. The inner belt contains high concentrations of electrons in the range of hundreds of keV and energetic protons with energies exceeding 100 MeV, trapped by the strong (relative to the outer belts) magnetic fields in the region.[55]

It is believed that proton energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms.[56]

Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the South Atlantic Anomaly.[57][58]

The proton belts contain protons with kinetic energies ranging from about 100 keV (which can penetrate 0.6 microns of lead) to over 400 MeV (which can penetrate 143 mm of lead).[59]

The PAMELA experiment detected orders of magnitude higher levels of antiprotons than are expected from normal particle decays while passing through the SAA. This suggests the van Allen belts confine a significant flux of antiprotons produced by the interaction of the Earth's upper atmosphere with cosmic rays.[60] The energy of the antiprotons has been measured in the range from 60 - 750 MeV.

When cosmic-ray protons enter the Earth’s atmosphere they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of billions of lighter particles, a so-called air shower.

An air shower is an extensive (many kilometres wide) cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic-ray proton (i.e. one of extraterrestrial origin) enters the atmosphere.

During solar proton events, ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region. In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours.

Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours.

"Because the Solar Wind Spectrometer made continuous measurements, it was possible to measure how the Earth's magnetic field affects arriving solar wind particles. For about two-thirds of each orbit, the Moon is outside of the Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter, with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as the magnetosheath, where the Earth's magnetic field affects the solar wind but does not completely exclude it. In this region, the particle flux is reduced, with typical proton velocities of 250 to 450 kilometers per second. During the lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar wind particles were measured."[61]

In February 2009, the ESA SARA LENA instrument aboard India's Chandrayaan-1 detected hydrogen ENAs sputtered from the lunar surface by solar wind protons. Predictions had been that all impacting protons would be absorbed by the lunar regolith but for an as yet unknown reason, 20% of them are bounced back as low energy hydrogen ENAs. It is hypothesized that the absorbed protons may produce water and hydroxyls in interactions with the regolith.[63][64]

This is an image of the alpha particle X-ray spectrometer (APXS). Credit: NASA/JPL-Caltech.

Some of the alpha particles are absorbed by the atomic nuclei. The [alpha,proton] process produces protons of a defined energy which are detected. Sodium, magnesium, silicon, aluminium and sulfur can be detected by this method. This method was only used in the Mars Pathfinder APXS.

"The Relativistic Proton Spectrometer (RPS) [measures] inner radiation belt protons with energies from 50 MeV-2 GeV. Such protons are known to pose a number of hazards to humans and spacecraft, including total ionizing dose, displacement damage, single event effects, and nuclear activation. The objectives of the investigation are to: (1) support the development of a new AP9/AE9 standard radiation model for spacecraft design; (2) to develop and test the model for RBSP data in general and RPS specifically; and, (3) to provide standardized worst-case specifications for dose rate, internal and deep dielectric chargins, and surface charging."[66]

At left is a diagram of one of the Van Allen Probes labeling various subsystems including the ECT.

"In addition to observing gamma rays from a solar flare, [ the Imaging Compton Telescope] COMPTEL is also capable of detecting solar neutrons. Neutron interactions within the instrument occur when an incident solar neutron elastically scatters off a hydrogen nucleus in the liquid scintillator of an upper D1 module. The scattered neutron may then interact and deposit all or a portion of its energy in one of the lower D2 modules, providing the internal trigger signal necessary for a double scatter event. The energy of the scattered neutron is deduced from its time of flight from the upper to lower detector, which is summed with the energy measured for the recoil proton in the upper D1 module to obtain the energy of the incident solar neutron. The computed scatter angle of the neutron, as with gamma rays, yields an event circle on the sky, which can be further constrained since the true source of the detected neutrons is assumed to be the Sun."[67]

A technician stands next to one of the twin Helios spacecraft during testing. Credit: NASA/Max Planck.

"Helios 1 and Helios 2 are a pair of probes launched into heliocentric orbit for the purpose of studying solar processes. On board, each probe carried an instrument for cosmic radiation investigation (the CRI) for measuring protons, electrons, and X-rays to determine the distribution of cosmic rays.

Explorer 45 (1971-096A) is a "NASA Small Scientific Satellite, S3-A ..., launched [using a Scout rocket on November 15, 1971,] from [the San Marco Platform,] Kenya, Africa, into an elliptical orbit having an apogee of 5.24 RE [27,031 km], a perigee of 220 km, an inclination of 3.5°, [eccentricity of 0.66982,] and a period of 7.82 hours. ... [carrying] two proton detectors and a three-axis fluxgate magnetometer. ... The lower energy proton instrument consists of an electrostatic analyzer-channeltron configuration measuring proton energies from 0.8 to 30 keV in 16 energy intervals."[68] Data stream ended on September 30, 1974,[69] and the satellite re-entered Earth's atmosphere on January 10, 1992.[70]

The stopping power of aluminum for protons is plotted versus proton energy. Credit: H.Paul.

At right, the figure shows the stopping power of aluminum metal single crystal for protons.

"Choosing materials with the largest stopping powers enables thinner detectors to be produced with resulting benefits in radiation tolerance (which is a bulk effect) and lower leakage currents. Alternatively, choosing smaller stopping powers will increase scattering efficiency, which is a requirement for polarimetry, or say, the upper detection plane of a double Compton telescope."[71]

"The higher energy proton detector [on Explorer 45] is a telescope detector system consisting of two surface barrier solid-state detectors behind a 2.2-kilogauss magnet used to sweep out electrons of energy less than 300 keV."[68] "This telescope measured the flux of protons in six channels covering the energy range 24.3 to 300 keV."[72]

"The heavy ion telescope [Explorer 45] had detectors of thicknesses 3.4 and 100 micrometers. This telescope uniquely identified the presence of protons, alpha particles (Z=2), and two groups of heavier ions, (Li,Be,B) and (C,N,O), plus ions with Z>=9. The heavy ion telescope measured proton fluxes in six channels covering the energy range 365 to 872 keV, and the fluxes of alpha particles in the energy ranges 1.16 to 1.74 keV and 1.74 to 3.15 keV. It measured the fluxes of Li, Be, and B ions in the ranges 3.6 to 7.1 MeV, 6.1 to 9.7 MeV, and 8.7 to 12.2 MeV, respectively, and the fluxes of C, N, and O ions in the ranges 12.1 to 15.7 MeV, 15.6 to 19.2 MeV, and 19.1 to 22.7 MeV, respectively. And it measured the flux of Z>=9 ions with energies > 20 MeV."[72]

"It has recently been suggested by Cane et al. [2002] that a class of type III solar radio bursts, called type III-l, is reliably associated with intense solar energetic particle (SEP) events. [...] we examine the durations, intensities, and other characteristics of such radio bursts in the hectometric frequency range and compare them to several groups of control events. We conclude that simple criteria, based on hectometric data alone, can identify the majority (∼80%) of type III-l radio bursts, which are associated with >20 MeV SEP proton events, while excluding almost 100% of the control events."[73]